Can seismic amplitude anomalies in the Campine Basin be related to geomanifestations?

Figure 1: Some examples of seismic amplitude anomalies which have been observed on recently acquired seismic data in northeastern Flanders. A: High amplitude anomaly, which seems to be related to a fault system; B: High amplitude anomaly in combination with a pull-down effect and a shadow zone; C: Polarity reversal in an anticlinal structure

Due to the thick overburden by Late Cretaceous and Cenozoic sediments, geomanifestations are not often present at the surface, but rather restricted to the subsurface of Flanders. One way to visualize geomanifestations in the subsurface is by the use of seismic data. Luckily, northeastern Flanders is covered by a dense network of 2D seismic lines, several of which were recently acquired due to the upcoming geothermal interest in the region. During the study of these recent, good quality lines, seismic amplitude anomalies were locally observed, often in association with faults (Fig. 1). Seismic amplitude anomalies are distinct expressions on a seismic image that may be caused by abrupt changes in geophysical contrasts (in density and velocity) in the subsurface. These abrupt changes in geophysical contrasts can represent geomanifestations, such as local cemented layers (higher density and velocity) or gas accumulations (lower density and velocity). The latter was shown in the blog of Johan ten Veen (2018) where several seismic amplitude anomalies in the offshore Netherlands were related to the presence of shallow gas. A seismic amplitude anomaly can, however, often be explained by more than one geomanifestation (c.f. Vernengo et al., 2017) and sometimes it can be attributed to non-geological processes (e.g. acquisition or processing issues). Therefore, the anomalies have to be thoroughly examined in order to establish whether they actually represent a geomanifestation and which type it could be. The most reliable way to examine this would be drilling a well into the seismic amplitude anomaly. As drilling wells is very expensive, other, more indirect techniques have been developed. At least two of these indirect techniques will be employed within Geoconnect3D to examine the seismic amplitude anomalies and establish a link with the structural framework (i.e. observed faults):

AVO analysis (Amplitude Variation with Offset): Analyzing lateral changes of amplitude trends with offset (the distance from shotpoint to receiver during seismic data acquisition). The amplitude variation with offset will depend mainly on lithology, but also the pore volume and its content (gas, oil or water). The AVO analysis can therefore help to establish whether and which type of fluid or gas caused the seismic amplitude anomaly.

Seismic inversion: In order to support the AVO analysis and to improve our understanding of seismic amplitude anomalies, it is important to be able to translate the seismic signal into (quantitative) rock properties. This method is known as seismic inversion. Well data reveal these rock properties and are therefore needed to perform this exercise. Consequently, the initial focus of this study will be on seismic amplitude anomalies in the vicinity of wells.

Although AVO analysis and seismic inversion are generally performed for oil and gas exploration on 3D seismic data, which are unfortunately only scarcely available in the Campine Basin, this project provides an opportunity to evaluate whether the available 2D seismic data are suitable to obtain unambiguous results from such types of analyses.

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